Fuel reforming system and control therefor

ABSTRACT

A fuel reforming system has a reformer ( 4 ) for reforming a fuel to produce a reformate gas; a shift converter ( 5 ) for reacting carbon monoxide (CO) contained in a reformate gas with water to produce hydrogen (H 2 ); a CO oxidizer ( 6 ) which removes CO discharged from the shift converter; and a startup combustor ( 11 ) for supplying combustion gas to the reformer ( 4 ), a shift converter ( 5 ) and CO oxidizer ( 6 ) to warm up them. An NOx trap ( 16 ) is disposed downstream of the startup combustor so as to adsorb nitrogen oxides (NOx) in combustion gas. A fuel reforming system further has a controller for controlling warm-up operations. When the warm-up operation for the reformer, the shift converter and the CO oxidizer is completed, the reformate reactions are commenced in the reformer ( 4 ). The NOx trapped by the NOx trap is decomposed by the reformate gas which contains CO gas and H 2  gas.

FIELD OF THE INVENTION

This invention relates to a fuel reforming system. In particular, itrelates to a fuel reforming system provided with an NOx trap foradsorbing nitrogen oxides (NOx).

BACKGROUND OF THE INVENTION

A startup combustor is sometimes used in order to warm up a reformer ofa fuel reforming system. The startup combustor preferably combusts fuelat approximately a stoichiometric ratio due to the large amount ofproduced heat. However since the combustion temperature is high whencombustion operations are performed at approximately a stoichiometricratio, NOx is produced largely as a result of reactions between oxygenand nitrogen in the air. Consequently there is the possibility that NOxcomponents will be present in exhaust emissions from the fuel reformingsystem. Furthermore when the fuel reforming system is applied to a fuelcell, entry of NOx into the fuel cell constitutes a cause ofdeterioration or contamination of the fuel cell. One method ofpreventing the production of NOx comprises control of the combustiontemperature in the combustor to a temperature at which NOx is notproduced.

A conventional technique disclosed in Tokkai Hei 9-063619 published bythe Japanese Patent Office in 1997 sets the excess-air factor (air-fuelratio/stoichiometric ratio) of the startup combustor to a value greaterthan one, for example to a value of three. Consequently the combustiontemperature is suppressed because of the excess air amount introducedinto the startup combustor.

SUMMARY OF THE INVENTION

However this method results in difficulties in maintaining flamecharacteristics and does not obtain stable startup combustion.Alternatively, although it is possible to suppress the temperature inthe combustor by suppressing the amount of fuel used in oxidizingreactions, this has the disadvantage that the amount of heat requiredfor starting the reformer is not produced.

It is therefore an object of this invention to advance startupoperations in the fuel reforming system while removing NOx in the fuelreforming system.

In order to achieve above objects, this invention provides a fuelreforming system having a reformer for reforming fuel to produce areformate gas containing hydrogen (H₂) gas and carbon monoxide (CO) gas;a shift converter for reacting carbon monoxide contained in thereformate gas with water (H₂O) to produce hydrogen; a CO oxidizer whichremoves CO gas from the reformate gas discharged from the shiftconverter and supplies the reformate gas to equipment using hydrogengas; a startup combustor for supplying combustion gas to the reformer,the shift converter and the CO oxidizer so as to warm-up the fuelreforming system; and a gas passage allowing flow of combustiongas/reformate gas, the gas passage extending to the equipment usinghydrogen gas from the startup combustion through the reformer, the shiftconverter and the CO oxidizer. The fuel reforming system comprises afirst fuel supply device for supplying fuel to the startup combustor; asecond fuel supply device for supplying fuel to the reformer; an airsupply device for supplying air to the startup combustor; a spark plugfor igniting fuel supplied to the startup combustor; an NOx trapdisposed between the startup combustor and the equipment using hydrogengas, the NOx trap adsorbing nitrogen oxides in the combustion gas; and acontroller for controlling a warm-up operation of the fuel reformingsystem.

The controller has functions of commanding the air supply device tosupply air to the startup combustor; commanding the first fuel supplydevice to supply fuel to the startup combustor; commanding the sparkplug to ignite fuel in the startup combustor so as to initiatecombustion in the startup combustor; and commanding the first fuelsupply device to stop supplying fuel to the startup combustor after afirst predetermined time has elapsed after initiating combustion, andthen commanding the second fuel supply device to supply fuel to thereformer.

The details as well as other features and advantages of this inventionare set forth in the remainder of the specification and are shown in theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a fuel reforming system according to afirst embodiment.

FIG. 2 is a flowchart showing a control routine executed by a controlleraccording to a first embodiment.

FIG. 3 is a schematic diagram showing a fuel reforming system accordingto a second embodiment.

FIG. 4 is a flowchart showing a control routine executed by a controlleraccording to a second embodiment.

FIG. 5 is a graph showing the relationship of the combustion temperatureto the air fuel ratio, the relationship of the discharge amount of NOxto the air fuel ratio, and the relationship of the overall dischargeamount of H2 and CO to the air fuel ratio.

FIG. 6 is a flowchart showing a control routine executed by a controlleraccording to a third embodiment.

FIG. 7 is a schematic diagram showing a fuel reforming system accordingto a fourth embodiment.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1, a first embodiment will be described. The fuelreforming system 70 is provided with a fuel tank 1, a reformer 4, ashift converter 5, a CO oxidizer 6 and a startup combustor 11. In thisdescription, the reformer 4, the shift converter 5 and the CO oxidizer 6are simply referred to as “reactors” in some cases.

The fuel tank 1 stores a fuel such as a hydrocarbon (e.g. gasoline) oran alcohol (e.g. methanol). The reformer 4 reforms fuel using reactionsbetween fuel and water in order to produce a reformate gas mainlycomprising carbon monoxide (CO), hydrogen (H₂) gas and carbon dioxide(CO₂). The shift converter 5 reacts water (H₂O) with CO gas in thereformate gas discharged from the reformer 4 to produce hydrogen andcarbon dioxide. The CO oxidizer 6 oxidizes and selectively removes COfrom the hydrogen-rich reformate gas which is discharged from the shiftconverter 5 and supplies the resulting reformate gas to the fuel cellstack 41. This example of a fuel cell stack 41 requires a supply ofreformate gas to generate power. However the CO oxidizer 6 may alsosupply gas to other equipment requiring reformate gas. The CO oxidizer 6is a preferential oxidation reactor. The startup combustor 11 suppliescombustion gas to the reformer 4, the shift converter 5 and the COoxidizer 6 which are disposed downstream from the startup combustor 11.

The fuel reforming system is provided with a gas passage 31 allowingflow of combustion gas from the startup combustor 11 to the fuel cellstack 41 via the reformer 4, the shift converter 5, and the CO oxidizer6 and flow of reformate gas from the reformer 4 to the fuel cell stack41.

The fuel reforming system 70 is provided with a first fuel supply device81 for supplying fuel from the fuel tank 1 to the startup combustor 11,a second fuel supply device 82 for supplying fuel from the fuel tank 1to the reformer 4, a first air supply device 83 for supplying air fromthe outside of the fuel reforming system to the startup combustor 11, asecond air supply device 84 for supplying air to the reformer 4 from theoutside of the fuel reforming system and a water supply device 85 forsupplying water to the reformer 4 from a water tank (not shown).

The first fuel supply device 81 is provided with a pump 12 which pumpsfuel from a fuel tank 1 and a fuel injector 13 which injects the fuel tothe startup combustor 11. The second fuel supply device 82 is providedwith a pump 2 which pumps fuel from a fuel tank 1 and a fuel injector 3which injects the fuel to the reformer 4. The first air supply device 83is provided with an air compressor 14 which supplies air whilecontrolling the flow amount of air. The second air supply device 84 isprovided with a flow control valve 10 which controls the flow amount ofair and an air compressor 9 which supplies air. The water supply device85 is provided with a pump 7 which pumps water from the water tank (notshown) and a water injector 8 which injects water to the reformer 4.

The controller 20 is provided with a microcomputer comprising a centralprocessing unit (CPU), a read only memory (ROM), a random access memory(RAM) and an input/output interface (I/O interface). The controller 20controls the flow control valve 10, the fuel injector 3, the waterinjector 8 so as to regulate the flow amount of water, air and fuelsupplied to the reformer 4. The controller 20 also controls the pump 12,the fuel injector 13, the air compressor 14 and the spark plug 15 basedon the temperature of various reactors such as the reformer 4 and theload condition of the fuel cell stack 41.

When reforming gasoline, the generally applicable temperature range forthe reformer, the shift converter and the CO oxidizer is respectivelygreater than or equal to 650° C., 200 to 450° C. and 80 to 250° C.

Fuel reforming systems which frequently perform startup and stopping ofthe system such as an automobile fuel reforming system encounterconsiderable problems in increasing the temperature of respectivereformers as rapidly as possible to the applicable temperature range.One method of increasing the temperature of the reactor rapidlycomprises producing a large amount of combustion heat in the startupcombustor 11 provided in the fuel reforming system 70 and supplying theheat to a downstream reactor.

As shown in FIG. 1, fuel from the fuel tank 1 is supplied to the startupcombustor 11 via a pump 12 and a fuel injector 13. Air is supplied by anair compressor 14. Fuel is combusted in the startup combustor 11 byigniting the fuel with a spark plug 15. The combustion gas produced inthe startup combustor 11 is supplied to a downstream reformer 4, theshift converter 5 and the CO oxidizer 6 for warm-up operations.

When the resulting amount of heat is excessive, the interior of thestartup combustor 11 will undergo excessive heating resulting in theproduction of nitrogen oxides (NOx). As a result, the conventionaltechnique suppresses the combustion temperature to a temperature atwhich NOx is not produced.

In order to eliminate the above restriction on the combustiontemperature, this invention provides an NOx trap 16 for adsorbing (orabsorbing) NOx. The NOx trap 16 is provided upstream of the fuel cellstack 41, preferably upstream of the CO oxidizer 6 and it comprises anNOx trap material which adsorbs NOx. In the first embodiment, the NOxtrap 16 is provided between the reformer 4 and the shift reactor 5. TheNOx trap 16 adsorbs NOx included in the combustion gas to warm up thereformer 4. The adsorbed NOx is reduced and decomposed by using gasesdisplaying reducing characteristics in the reformate gas producedimmediately after completion of warm up, that is to say, by using carbonmonoxide (CO) gas and hydrogen (H₂) gas. In this manner, the startupcombustor 11 is allowed to perform combustion operations in atemperature range in which NOx would be produced. Normally the thresholdtemperature which is the lower limiting temperature of the temperaturerange in which NOx is produced is approximately 950° C. irrespective ofthe type of fuel used.

Since an NOx trap is provided so as to adsorb NOx in the the combustiongas or reformate gas downstream of the startup combustor 11 in the fuelreforming system 70, NOx produced by combustion in the startup combustor11 at temperatures greater than about 950° C. can be eliminated.Likewise, at temperatures of less than about 950° C., NOx produced dueto localized increases in the combustion temperature resulting fromnon-uniform combustion can be eliminated by the NOx trap 16 from thecombustion gas discharged from the startup combustor 11. Since NOxadsorbed by the NOx trap 16 can be decomposed by carbon monoxide andhydrogen in the reformate gas produced immediately after completion ofthe warm-up operation, the reformer 4 can be rapidly started usinghigh-temperature combustion gas without resulting in NOx emissions tothe fuel cell stack 41.

Now the NOx trap 16 will be described in detail below.

The NOx trap 16 is provided with an NOx trap material which traps NOx.The NOx trap material comprises a precious metal or a transition metal(or both) as a catalyst. Furthermore, the NOx trap material comprises analkali metal or an alkaline-earth metal, (or both) as an NOx adsorbent.The precious metal may be platinum (Pt), rhodium (Rh), paladium (Pd) orruthenium (Ru). The transition metal may be copper (Cu), nickel (Ni), orcobalt (Co). The alkali metal may be lithium (Li), sodium (Na), kalium(K) or cesium (Cs). The alkaline-earth metal may be magnesium (Mg),calcium (Ca), strontium (Sr) or barium (Ba).

The NOx adsorption mechanism is as follows: (a) NO, NO₂ molecules areadsorbed on the surface of the transition metal or the precious metal;(b) the NOx which has become highly oxidized by the oxidizing effect ofthe transition metal or the precious metal adheres to the surface of theprecious metal or the transition metal; (c) the NOx which has becomehighly oxidized with x larger than 2 by the oxidizing effect of thetransition metal or the precious metal undergoes ion-mediated bondingwith the alkali or alkaline-earth metal, for example in the form ofK(NO₃) or Ba(NO₃)₂.

According to spectroscopic analysis of the adhering NOx using a FourierTransform Infrared. Spectrometer (FTIR), several adsorbed species of NOxexist, depending on the temperature of the NOx trap material, theconditions on the surface of the NOx trap material and the concentrationof oxygen in the surrounding gases. Thus, considerable difficulty isencountered in specifying the state of such NOx.

In the temperature range of about 100° C. to 550° C., the NOx trapmaterial can adsorb NOx. At a temperature less than approximately 100°C., adsorption of NOx on the surface of the NOx trap material ishindered due to the presence of condensed water on the surface of theNOx trap material. At a temperature greater than 550° C., the thermalenergy in the adsorbed NOx exceeds the adsorption energy of NOx withrespect to the surface of the NOx trap material, resulting indissociation of NOx from the surface. Furthermore species ofion-mediated adsorption. (e.g. NO₃—) become thermally unstable and isdischarged as NO or NO₂.

The adsorbed NOx is reduced and decomposed by a reducing gas containedin the reformate gas, that is to say, by hydrogen (H2) gas or carbonmonoxide (CO) gas. Since NOx is adsorbed in various ways, numerousreaction pathways are present. An example of a reducing reaction for NOxis shown below.NO₂+2CO→½N₂+2CO₂NO₂+2H₂→½N₂+2H₂O

The present inventors have experimentally confirmed that these reactionsare promoted at greater than 150° C.

The decomposition of the adsorbed NOx using reduction reactions in thefuel reforming system 70 will be described in detail below.

The total amount of H₂ gas and CO gas for decomposing adsorbed NOx is asubstantially fixed value from the outlet of the reformer 4 to the inletof the CO oxidizer 6. This is due to the fact that theoretically CO gasproduced in the reformer 4 is transformed into CO₂ gas while producingan equal number of moles of H₂ via a shift reaction. When a normal fuelis used, the total concentration of H₂ gas and CO gas in the reformategas for reducing NOx adsorbed by the NOx trap material is 25–40% vol.

When an internal combustion engine is provided with a known NOx trapcatalyst for purifying exhaust gas, the total concentration of H₂ gasand CO gas produced during rich-spike operations for decomposingadsorbed NOx is less than equal to 10% vol in the exhaust gas. This isdue to the fact that performing rich-spike operations is difficult at anexcess-air factor λ (=air-fuel ratio/stoichiometric ratio) of less than0.7. The low concentration of H₂ gas and CO gas in the exhaust gas leadsto insufficient decomposition of NOx when an internal combustion engineuses an NOx trap catalyst to purify exhaust gas. Therefore under normalconditions, 5–20% of the NOx flowing into the NOx trap catalyst isdischarged downstream of the NOx trap catalyst.

Conversely, the concentration of gaseous reductant (CO and H₂) in thereformate gas produced by the reformer is 2–4 times larger than that inexhaust gas discharged from the internal combustion engine. When the NOxtrap 16 is provided in the fuel reforming system 70, it is possible toobtain a decomposition efficiency for NOx which is approximately greaterthan 99%.

As shown above, startup operations for a fuel reforming system 70 arecontrolled by a controller 20. The flowchart shown in FIG. 2 describes acontrol routine executed during startup of a fuel reforming system 70.In the control routine, each step comprises one or more instructionswhich the controller 20 provides.

Firstly in a step S1, combustion is started in the startup combustor 11.Here, the pump 12 is commanded to pump fuel from the fuel tank 1 and thecompressor 14 is commanded to supply air to the startup combustor 11.The fuel injector 13 is commanded to inject fuel into the startupcombustor 11. A spark plug 15 is command to ignite the fuel. As aresult, the fuel is combusted in the startup combustor 11. Here, theinitial value of the variable T expressing an elapsed time is set tozero. At this time, the supplied amounts of fuel and air are regulatedby the controller 20 so that the temperature of combustion is higherthan the threshold temperature above which NOx is produced. Thethreshold temperature takes a value of approximately 950° C.irrespective of the type of fuel. For example, the combustiontemperature is controlled to a target combustion temperature of 1050° C.which is 100° C. higher than the threshold temperature. From the pointof view of air fuel ratio control, the flow amount of air and fuel iscontrolled in order to realize a lean air fuel ratio A1 which canrealize the target combustion temperature of 1050° C. shown by the graphin FIG. 5. Referring to FIG. 5, at the lean air fuel ratio A1, CO and H2are substantially not present in the combustion gas.

Thus high-temperature combustion gas is introduced into the reformer 4to rapidly increase the temperature of the reformer 4. Since thetemperature of the combustion gas is controlled to a temperature (e.g.1050° C.) higher than the threshold temperature as described above,reactions occur between oxygen and nitrogen in the air resulting in theformation of NOx. However such NOx is eliminated by the NOx trap 16provided downstream of the startup combustor 11 and thus NOx can beprevented from being discharged downstream of the NOx trap 16.

Then in a step S2, a variable T expressing time is incremented by avalue of one. The variable T measures the elapsed time after theinitiation of warm-up operations in the step S1. The value of thevariable T is incremented by a value of one per unit time. When the unittime is one second, the counter T increases each second.

In a step S3, it is determined whether or not the variable T expressingtime is greater than a first predetermined time TA0. When the variable Tis smaller than or equal to the first predetermined time TA0, theroutine returns to a step S2. When the variable T is greater than thefirst predetermined time TA0, the routine proceeds to a step S4. Thefirst predetermined time TA0 represents the required time for warming upthe reformer 4, the shift converter 5 and the CO oxidizer 6 using thecombustion gas and it depends on factors such as the amount of heatproduced by the startup combustor 11, the heat capacity of the reactorsand startup combustor 11, and the target temperature of the inlet of theCO oxidizer 6. Above the target temperature, the CO oxidizer 6 cansuccessfully removes CO. For example, the first predetermined time TA0is about 90 seconds for the fuel reforming system which is used for afuel cell stack with a rated output of 65 kW and has the targettemperature of the inlet of the CO oxidizer 6 of 120° C. The firstpredetermined time TA0 is determined in advance by experiment and isstored in the memory of the controller 20.

When the warm-up operation for every reactor is completed (thepredetermined time TA0 has elapsed), the routine proceeds to a step S4where the supply of fuel and air to the startup combustor 11 is stopped.Here, the fuel injector 13 is commanded to stop injecting fuel and theair compressor 14 is commanded to stop its operation. Then in a step S5,air, water and fuel are supplied to the reformer 4 and reformatereactions are commenced. Here, the pumps 2, 7 and the air compressor 9are commanded to start operating, the flow control valve 10 is commandedto open, the fuel injector 3 is commanded to start injecting fuel to thereformer 21, and the water injector 8 is commanded to start injectingwater to the reformer 21.

Since the reformate gas produced by the reformer 4 is supplied to theNOx trap 16, NOx adsorbed by the NOx trap material of the NOx trap 16 isreduced and decomposed by the CO gas and H2 gas present in the reformategas.

In a step S6, the variable T is initialized to a value of zero and thecontrol routine is terminated.

Referring to FIG. 3, a second embodiment of this invention will bedescribed. In the first embodiment, warm-up operations for reactors wereperformed using only the heat of combustion gas supplied from thestartup combustor 11. However in the second embodiment, in addition tousing the heat of combustion gases, warm-up operations for each reactorare performed by producing heat inside each reactor resulting fromoxidation reactions of CO and H₂ in the combustion gas with oxygen. Thecombustion gas from the startup combustor 11 can contain gaseous CO andH₂ by enriching the air fuel ratio in the startup combustor 11. Sinceidentical components are designated by the same reference numerals inthe first and the second embodiments, additional description will beomitted with respect to these components.

In this embodiment, an NOx trap comprising an NOx trap material whichadsorbs NOx is provided in at least one of the reformer 4, the shiftconverter 5 and the CO oxidizer 6. In FIG. 3, an NOx trap comprising anNOx trap material is provided in all of the reformer 4, the shiftconverter 5 and the CO oxidizer 6. The NOx trap material may be mixedwith a catalyst of each reactor and disposed in each reactor. Thus sinceeach reactor has an NOx trapping function, reactions of gaseous CO andH₂ with oxygen in the reactors can be initiated at a lower temperaturethan when the reactors do not trap NOx. This is due to the fact that thestartup temperature of oxidizing reaction is lower when NOx (x>2) isadsorbed in the NOx trap material than when oxygen is present only ingaseous phase. In contrast to normal gaseous oxygen, oxygen contained inthe adsorbed NOx (x>2) is activated because N—O bonding is weak due tothe interaction of NOx with electrons on the surface of the NOx trapmaterial. Thus the adsorbed NOx serves as a source of activated oxygen.Furthermore NOx is generally unstable when the value “x” for oxygen islarge and oxygen displays a tendency to disassociate. Consequently suchforms of NOx act as strong oxidizing agents. For example, the nitrateion NO₃ ⁻ bonded with the NOx adsorbent acts as a strong oxidizingagent.

On the other hand, apart from warming up using the heat of combustiongas, there is the method of warming up reactors by introducing gaseousCO and H₂ into the reactors and introducing air containing a requiredamount of oxygen for oxidizing H₂ and CO in the respective reactors.

Thus the reactor is rapidly warmed by the heat of oxidizing reactionsoccurring in the reactor. First, CO and H₂ contained in the combustiongas react with oxygen using NOx as an oxidizing agent. Subsequently, COand H₂ contained in the combustion gas react with oxygen using air as anoxidizing agent. The air compressor 24 supplies air through a flowcontrol valve 25 to the shift converter 22 and the CO oxidizer 23.Starting oxidizing reactions at lower temperatures due to the presenceof adsorbed NOx acting as a source of activated oxygen leads to rapidwarming up operations.

Referring to the flowchart in FIG. 4, a control routine executed by thecontroller 20 according to a second embodiment will be described.

Firstly in a step S11, combustion is started in the startup combustor11. Here, the pump 12 is commanded to supply fuel from the fuel tank 1and the compressor 14 is commanded to supply air to the startupcombustor 11. The fuel injector 13 is commanded to inject fuel into thestartup combustor 11. This fuel is ignited by a spark plug 15. In thismanner, the fuel is combusted. Here, the initial value of the variable Texpressing an elapsed time is set to zero. At this time, the air fuelratio is controlled to an air fuel ratio of A1 as shown in FIG. 5. Atthe air fuel ratio A1 is lean, the temperature of combustion is higherthan 950° C.

Thus high-temperature combustion gas is introduced into the reformer 21and the temperature of the reformer 21 is rapidly increased by the heatof the combustion gas. Since the combustion temperature in the startupcombustor 11 is high, reactions occur between oxygen and nitrogen in theair resulting in the formation of NOx in the combustion gas. Howeversuch NOx is reduced by the reformer 21, shift converter 22 and COoxidizer which have an NOx adsorbing function and are provideddownstream of the startup combustor 11. Thus it is possible to preventNOx from being discharged to the fuel cell stack 41.

Then in a step S12, a variable T expressing time is incremented by avalue of one. In a step S13, the elapsed time T after the initiation ofstartup operations in a step S11 is compared with a second predeterminedtime TB0. When it is determined that the elapsed time T is smaller thanthe predetermined time TB0, the routine returns to a step S12. When theelapsed time T is greater than the second predetermined time TB0, theroutine proceeds to a step S14. The value of TB0 is determined inadvance by experiment and is stored in the memory of the controller 20.The second predetermined time TB0 represents the time until reaching atemperature at which the reformer 21, the shift converter 22 and the COoxidizer 23 which have adsorbed NOx can oxidize H2 and CO. If eachreactor is not provided with an NOx trap material, the time TB1 for allreactors to reach their respective temperature at which oxidizingreactions of H2 and CO are enabled is longer than when each reactor isprovided with an NOx trap material. The time TB1 is stored in the memoryof the controller 20 and used as a third predetermined time TB1 in thestep S17 below. The time TB1 depends on factors such as the amount ofheat produced by the startup combustor 11, the heat capacity of thereactors and startup combustor 11, and the target temperature of theinlet of the CO oxidizer 6. The time TB1 is about 70 seconds for thefuel reforming system used for a fuel cell stack with a rated output of65 kW.

In a step S14, the combustion state of the startup combustor 11 isswitched to a rich combustion state at an air fuel ratio B1 as shown inFIG. 5 from the lean air fuel ratio A1. Here, the controller 20 controlsthe amounts of fuel and air supplied to the startup combustor 11 usingthe fuel injector 13 and the air compressor 14. In this manner, thesupply amount of H₂ gas and CO gas to the startup combustor 11 isincreased. The NOx adsorbed by the NOx trap material functions as asource of activated oxygen and produces reactions of H₂ and CO withoxygen on the NOx trap material.

In a step S15, the air compressor 9 and the flow control valve 10 arecommanded to start supplying air to the reformer 21, and the aircompressor 24 and the flow control valve 25 are commanded to startsupplying air to the shift converter 22 and the CO oxidizer 23. Here,the oxidizing reactions are further promoted in each reactor bysupplying air from a section upstream of the respective reactors inorder to further increase the heat of oxidizing reactions.

If NOx was not adsorbed in each reactor due to absence of the NOx trapmaterial, the temperature of each reactor at this step would be atemperature at which oxidation reactions do not occur. However in thisembodiment, since NOx is adsorbed on the NOx trap material in eachreactor, the minimum temperature for producing oxidizing reactions isreduced by about 20° C. Though the minimum temperature for producingoxidizing reactions between H₂ and gaseous O₂ is about 120° C., theminimum temperature for producing oxidizing reactions between H₂ andoxygen of the adsorbed NOx is about 100° C. Likewise, though the minimumtemperature for producing oxidizing reactions between CO and gaseous O₂is about 150° C., the minimum temperature for producing oxidizingreactions between CO and oxygen of the adsorbed NOx is about 130° C. Asa result, in the step S15, oxidizing reactions start in each reactor.This allows the warm-up time for each reactor to be shortened. When thefuel reforming system 70 performs reformate operations, thedecomposition of NOx is promoted irrespective of the operatingconditions.

In a step S16, the variable T expressing an elapsed time is incrementedby a value of one. In a step S17, it is determined whether or not theelapsed time T after initiating the warm-up operations in the step S11is greater than a third predetermined time TB1. When the elapsed time Tis smaller than or equal to the third predetermined time TB1, theroutine returns to the step S16. When the elapsed time T is greater thanthe third predetermined time TB1, the routine proceeds to a step S18. Inthe step S18, since the warming up operations for each reactor arecompleted, the supply of air and fuel to the startup combustor 11 isstopped. Here, the fuel injector 13 is commanded to stop injecting fueland the air compressor 14 is commanded to stop supplying air. The supplyof air to each reactor 21, 22, 23 is also stopped by commanding the flowcontrol valve 10 and the flow control valve 25 to close.

Next, in a step S19, the required amounts of fuel, air and water forreformate operations is supplied to the reformer 21 and reformingoperation are initiated. Here, the pumps 2, 7 and the air compressor 9are commanded to start operating, the flow control valve 10 is commandedto open, the fuel injector 3 is commanded to start injecting fuel to thereformer 21, and the water injector 8 is commanded to start injectingwater to the reformer 21.

During reforming operations, residual NOx on the NOx trap material isdecomposed by the H₂ and CO produced by the reformer 21.

In a step S20 the variable T is initialized to a value of zero and thecontrol routine is terminated.

In the second embodiment, an NOx adsorbing function has been provided ineach reactor 21, 22, 23. However it is not required to provide all thereactors with this function and for example, only the reformer 21 may beprovided with the adsorbing function.

In the second embodiment, the controller 20 switches the combustionstate of the startup combustor 11 in response to the elapsed time afterstarting combustion, that is to say, in response to the NOx traptemperature which increases with the elapsed time. In this manner, it ispossible to prevent NOx emissions from the reforming system as a resultof NOx not being removed by adsorption. If each reactor is provided witha temperature sensor to measure the temperature of the NOx trap, thecontroller 20 may control the operation of the startup combustor 11 inresponse to the directly measured temperature of the NOx trap in thesimilar manner to the control routine shown in the routine in FIG. 4.

Referring to a flowchart in FIG. 6, a control routine executed by thecontroller 20 according to a third embodiment will be described. Thestructure of the fuel reforming system is the same as the firstembodiment.

The third embodiment has the object of reducing NOx emissions from thestartup combustor 11 immediately after starting the startup combustor11. At temperatures of approximately less than 100° C., moisture isadsorbed in the NOx trap. Consequently there is the possibility that theNOx trap will not be able to adsorb NOx in a preferred manner. Thus thestartup combustor 11 performs low-temperature combustion whichdischarges low amounts of NOx until the NOx trap reaches a predeterminedtemperature of approximately 100° C. After the NOx trap exceeds thepredetermined temperature of approximately 100° C., the startupcombustor 11 performs the same control routine as the first embodiment.

In a step S21, fuel and air are supplied to the startup combustor 11 andthe fuel is ignited by a spark plug 15 in a manner similar to the stepS1 and S11. Here, the initial value of the variable T expressing time isset to zero. Then in a step S22, the variable T is incremented by one.The air fuel ratio at this time is controlled to coincide with the airfuel ratio C1 shown in FIG. 5. It should be noted that the air fuelratio C1 is leaner than the air fuel ratio A1 with the result thatcombustion itself becomes unstable and difficult to control. Howeverunstable combustion is only performed for a fourth predetermined shorttime TC0. The fourth predetermined time TC0 represents the required timefor increasing the temperature of the NOx trap 16 to a temperature atwhich the NOx trap material eliminates condensed water, that is to say,the time that has elapsed after initiating startup operation until atemperature of the NOx trap 16 reaches approximately 100° C. The amountof NOx produced under these combustion conditions is small in comparisonto combustion at an air fuel ratio A1 due to the low combustiontemperature.

In a step S23, it is determined whether or not the variable T whichexpresses the elapsed time after initiating warm-up operations isgreater than the fourth predetermined time TC0. When the variable Twhich expresses the elapsed time is smaller than the fourthpredetermined time TC0, the routine returns to a step S22. When thevariable T which expresses elapsed time is greater than the fourthpredetermined time TC0, the routine proceeds to a step S1. Thesubsequent steps after S1 are the same as the steps S1–S6 in theflowchart in FIG. 2 which show the control routine according to thefirst embodiment. Therefore additional description will be omitted.

In this manner, until warm-up operations for the NOx trap material arecompleted, NOx emissions can be reduced in a fuel reforming system whichfurther reduces NOx emissions from the startup combustor 11 by usinglean combustion at an air fuel ratio C1.

In a third embodiment, the controller 20 switches the combustion stateof the startup combustor 11 in response to the elapsed time afterstarting combustion, that is to say, with increase in the temperature ofthe NOx trap so that NOx emissions from the reforming system 70 areprevented by adsorption. If each reactor is provided with a temperaturesensor to measure the temperature of the NOx trap, the controller 20 maycontrol the operation of the startup combustor 11 in response to thedirectly measured temperature of the NOx trap in a manner similar to thecontrol shown in the routine in FIG. 4.

Referring to FIG. 7, a fourth embodiment will be described. The fourthembodiment adds a second NOx trap 30 to the first embodiment. The secondNOx trap 30 is disposed between the startup combustor 11 and thereformer 4. That is to say, NOx traps are respectively providedimmediately downstream of the startup combustor 11 and immediatelydownstream of the reformer 4.

The first NOx trap 16 is provided between the reformer 4 and the shiftconverter 5. Immediately after initiating warm-up operations for thefuel reforming system 70, high-temperature combustion gas flows directlyinto the second NOx trap 30 provided immediately downstream of thestartup combustor 11. The second NOx trap 30 rapidly adsorbs the NOx. Atthe same time, the NOx trap material in the first NOx trap 16 which isdownstream of the second NOx trap 30 is warmed by combustion gas whichdoes not contain NOx. If the temperature of the second NOx trap 30exceeds a temperature of 500–550° C. which is the maximum temperaturebelow which NOx can be adsorbed, adsorbed NOx will be dischargeddownstream of the second NOx trap 30. Therefore the fuel reformingsystem 70 is adapted so that the NOx trap material of the first NOx trap16 reaches a temperature of greater than approximately 100° C. beforethe temperature of the second NOx trap 30 exceeds a temperature of500–550° C. At a temperature of approximately greater than 100° C.,since moisture adsorbed by the NOx trap material of the first NOx trap16 is eliminated, the first NOx trap 16 can remove NOx by adsorption. Inthis manner, NOx emissions from the reforming system 70 are effectivelyremoved using the two NOx traps. The NOx adsorbed in the second NOx trap30 is decomposed by the reformate gas produced by the reformer 4 as inthe case of the first NOx trap 16 after the completion of warm-upoperations because the gas passage 31 allows flow of the reformate gasfrom the reformer 4 to the second NOx trap 30.

In the above embodiments, a fuel reforming system 70 generates areformate gas rich in H₂ and supplies the reformate gas to the fuel cellstack 41. However the fuel reforming system is not limited to a fuelcell, and it is possible to supply reformate gas to a equipment using areformate gas rich in H₂.

This invention can clearly be applied to reforming systems withdifferent structures from that described in the embodiments above. Forexample, it is possible to apply this invention to a reforming systemwith a different combination of type of reactor.

The entire contents of Japanese Patent Application P2001-350997 (filedNov. 16, 2001) are incorporated herein by reference.

Although the invention has been described above by reference to certainembodiments of the invention, the invention is not limited to theembodiments described above. Modifications and variations of theembodiments described above will occur to those skilled in the art, inlight of the above teachings. The scope of the invention is defined withreference to the following claims.

INDUSTRIAL APPLICABILITY

This invention can be applied to a fuel reforming system which includesa source of NOx such as a combustor, resulting in a reduction of NOx ina reformate gas discharged from the fuel reforming system. The fuelreforming system according to this invention can supply hydrogen gaswithout NOx to equipment such as a fuel cell.

1. A fuel reforming system having a reformer for reforming fuel toproduce a reformate gas containing hydrogen (H₂) gas and carbon monoxide(CO) gas; a shift converter for reacting carbon monoxide contained inthe reformate gas with water (H₂O) to produce hydrogen; a CO oxidizerwhich removes CO gas from the reformate gas discharged from the shiftconverter and supplies the reformate gas to equipment using hydrogengas; a startup combustor for supplying combustion gas to the reformer,the shift converter and the CO oxidizer so as to warm-up the fuelreforming system; and a gas passage allowing flow of combustiongas/reformate gas, the gas passage extending to the equipment usinghydrogen gas from the startup combustor through the reformer, the shiftconverter and the CO oxidizer; the fuel reforming system comprising: afirst fuel supply device for supplying fuel to the startup combustor; asecond fuel supply device for supplying fuel to the reformer; an airsupply device for supplying air to the startup combustor; a spark plugfor igniting fuel supplied to the startup combustor; an NOx trapdisposed between the startup combustor and the equipment using hydrogengas, the NOx trap adsorbing nitrogen oxides in the combustion gas; and acontroller for controlling a warm-up operation of the fuel reformingsystem, the controller having functions of: commanding the air supplydevice to supply air to the startup combustor; commanding the first fuelsupply device to supply fuel to the startup combustor; commanding thespark plug to ignite fuel in the startup combustor so as to initiatecombustion in the startup combustor; controlling a flow amount of airsupplied to the startup combustor from the air supply device and a flowamount of fuel supplied to the startup combustor from the first fuelsupply device, in response to a temperature of the NOx trap; andcommanding the first fuel supply device to stop supplying fuel to thestartup combustor after a first predetermined time has elapsed afterinitiating combustion, and then commanding the second fuel supply deviceto supply fuel to the reformer.
 2. The fuel reforming system as definedin claim 1, wherein at least one member selected from the groupconsisting of the reformer, the shift converter and the CO oxidizercomprises the NOx trap.
 3. The fuel reforming system as defined in claim1, wherein the controller has the function of controlling the flowamount of air supplied to the startup combustor from the air supplydevice and the flow amount of fuel supplied to the startup combustorfrom the first fuel supply device at a first lean air fuel ratio (C1)until the NOx trap reaches a predetermined temperature, and at a secondlean air fuel ratio (A1) after the NOx trap reaches the predeterminedtemperature, wherein the first air fuel ratio (C1) is leaner than thesecond air fuel ratio (A1).
 4. The fuel reforming system as defined inclaim 3, wherein the predetermined temperature is substantially equal to100° C.
 5. The fuel reforming system as defined in claim 1, wherein theNOx trap is disposed in the gas passage between the reformer and theshift converter.
 6. The fuel reforming system as defined in claim 5,wherein another NOx trap is disposed in the gas passage between thereformer and the startup combustor.
 7. The fuel reforming system asdefined in claim 1, wherein the first predetermined time is the timerequired for warming up the shift converter, the reformer and the COoxidizer.
 8. The fuel reforming system as defined in claim 1, whereinnitrogen oxides adsorbed by the NOx trap are reduced by hydrogen (H₂)gas and carbon monoxide (CO) gas contained in the reformate gas.
 9. Thefuel reforming system as defined in claim 1, wherein the NOx trap isprovided with an NOx trap material which comprises at least one memberselected from the group consisting of a precious metal and a transitionmetal and which further comprises at least one member selected from thegroup consisting of an alkali metal and an alkali-earth metal.
 10. Afuel reforming system having a reformer for reforming fuel to produce areformate gas containing hydrogen (H₂) gas and carbon monoxide (CO) gas;a shift converter for reacting carbon monoxide contained in thereformate gas with water (H₂O) to produce hydrogen; a CO oxidizer whichremoves CO gas from the reformate gas discharged from the shiftconverter and supplies the reformate gas to equipment using hydrogengas; a startup combustor for supplying combustion gas to the reformer,the shift converter and the CO oxidizer so as to warm-up the fuelreforming system; and a gas passage allowing flow of combustiongas/reformate gas, the gas passage extending to the equipment usinghydrogen gas from the startup combustor through the reformer, the shiftconverter and the CO oxidizer; the fuel reforming system comprising: afirst fuel supply device for supplying fuel to the startup combustor; asecond fuel supply device for supplying fuel to the reformer; an airsupply device for supplying air to the startup combustor; a spark plugfor igniting fuel supplied to the startup combustor; an NOx trapdisposed between the startup combustor and the equipment using hydrogengas, the NOx trap adsorbing nitrogen oxides in the combustion gas; and acontroller for controlling a warm-up operation of the fuel reformingsystem, the controller having functions of: commanding the air supplydevice to supply air to the startup combustor; commanding the first fuelsupply device to supply fuel to the startup combustor; commanding thespark plug to ignite fuel in the startup combustor so as to initiatecombustion in the startup combustor; controlling a flow amount of airsupplied to the startup combustor from the air supply device and a flowamount of fuel supplied to the startup combustor from the first fuelsupply device, in response to the elapsed time after initiation ofcombustion in the startup combustor; and commanding the first fuelsupply device to stop supplying fuel to the startup combustor after afirst predetermined time has elapsed after initiating combustion, andthen commanding the second fuel supply device to supply fuel to thereformer.
 11. The fuel reforming system as defined in claim 10, whereinthe controller has the function of controlling the flow amount of airsupplied to the startup combustor from the air supply device and theflow amount of fuel supplied to the startup combustor from the firstfuel supply device so that a lean air fuel ratio is produced until asecond predetermined time elapses after the initiation of combustion bythe startup combustor, and so that a rich air fuel ratio is producedafter the second predetermined time elapses after the initiation ofcombustion by the startup combustor, the second predetermined time isshorter than the first predetermined time.
 12. The fuel reforming systemas defined in claim 11, wherein the second predetermined time is theelapsed time after the initiation of combustion by the startup combustoruntil a temperature of the NOx trap is reached to a temperature at whichnitrogen oxides adsorbed by the NOx trap can oxidize hydrogen (H₂) gasand carbon monoxide (CO) gas.
 13. The fuel reforming system as definedin claim 10, wherein the NOx trap is disposed in the gas passage betweenthe reformer and the shift converter.
 14. The fuel reforming system asdefined in claim 13, wherein another NOx trap is disposed in the gaspassage between the reformer and the startup combustor.
 15. The fuelreforming system as defined in claim 10, wherein the first predeterminedtime is the time required for warming up the shift converter, thereformer and the CO oxidizer.
 16. The fuel reforming system as definedin claim 10, wherein nitrogen oxides adsorbed by the NOx trap arereduced by hydrogen (H₂) gas and carbon monoxide (CO) gas contained inthe reformate gas.
 17. The fuel reforming system as defined in claim 10,wherein the NOx trap is provided with an NOx trap material whichcomprises at least one member selected from the group consisting of aprecious metal and a transition metal and which further comprises atleast one member selected from the group consisting of an alkali metaland an alkali-earth metal.
 18. A control method for controlling a fuelreforming system, the fuel reforming system having a reformer forreforming fuel to produce a reformate gas containing hydrogen (H₂) gasand carbon monoxide (CO) gas; a shift converter for reacting carbonmonoxide contained in the reformate gas with water (H₂O) to producehydrogen; a CO oxidizer which removes CO gas from the reformate gasdischarged from the shift converter and supplies the reformate gas toequipment using hydrogen gas; a startup combustor for supplyingcombustion gas to the reformer, the shift converter and the CO oxidizerso as to warm-up the fuel reforming system; an NOx trap disposed betweenthe startup combustor and the equipment using hydrogen gas; and a gaspassage allowing flow of combustion gas/reformate gas; the NOx trapadsorbing nitrogen oxides in the combustion gas, the gas passageextending to the equipment using hydrogen gas from the startup combustorthrough the reformer, the shift converter and the CO oxidizer; thecontrol method comprising: supplying fuel to the startup combustor;supplying air to the startup combustor; igniting fuel supplied to thestartup combustor; controlling a flow amount of air supplied to thestartup combustor from the air supply device and a flow amount of fuelsupplied to the startup combustor from the first fuel supply device, inresponse to a temperature of the NOx trap; stopping supplying fuel tothe startup combustor after a first predetermined time has elapsed afterinitiating combustion; and supplying fuel to the reformer.
 19. A fuelreforming system having a reformer for reforming fuel to produce areformate gas containing hydrogen (H₂) gas and carbon monoxide (CO) gas;a shift converter for reacting carbon monoxide contained in thereformate gas with water (H₂O) to produce hydrogen; a CO oxidizer whichremoves CO gas from the reformate gas discharged from the shiftconverter and supplies the reformate gas to equipment using hydrogengas; a startup combustor for supplying combustion gas to the reformer,the shift converter and the CO oxidizer so as to warm-up the fuelreforming system; and a gas passage allowing flow of combustiongas/reformate gas, the gas passage extending to the equipment usinghydrogen gas from the startup combustor through the reformer, the shiftconverter and the CO oxidizer; the fuel reforming system comprising: anNOx trap disposed between the startup combustor and the equipment usinghydrogen gas, the NOx trap adsorbing nitrogen oxides in the combustiongas; first means for supplying fuel to the startup combustor; secondmeans for supplying fuel to the reformer; third means for supplying airto the startup combustor; fourth means for igniting fuel supplied to thestartup combustor; and fifth means for commanding the third means tosupply air to the startup combustor; sixth means for commanding thefirst means to supply fuel to the startup combustor; seventh means forcommanding the fourth means to ignite fuel in the startup combustor soas to initiate combustion in the startup combustor; eighth means forcontrolling a flow amount of air supplied to the startup combustor fromthe air supply device and a flow amount of fuel supplied to the startupcombustor from the first fuel supply device, in response to atemperature of the NOx trap; ninth means for commanding the first meansto stop supplying fuel to the startup combustor after a firstpredetermined time has elapsed after initiating combustion, and tenthmeans for commanding the second means to supply fuel to the reformer.20. A control method for controlling a fuel reforming system, the fuelreforming system having a reformer for reforming fuel to produce areformate gas containing hydrogen (H₂) gas and carbon monoxide (CO) gas;a shift converter for reacting carbon monoxide contained in thereformate gas with water (H₂O) to produce hydrogen; a CO oxidizer whichremoves CO gas from the reformate gas discharged from the shiftconverter and supplies the reformate gas to equipment using hydrogengas; a startup combustor for supplying combustion gas to the reformer,the shift converter and the CO oxidizer so as to warm-up the fuelreforming system; an NOx trap disposed between the startup combustor andthe equipment using hydrogen gas; and a gas passage allowing flow ofcombustion gas/reformate gas; the NOx trap adsorbing nitrogen oxides inthe combustion gas, the gas passage extending to the equipment usinghydrogen gas from the startup combustor through the reformer, the shiftconverter and the CO oxidizer; the control method comprising: supplyingfuel to the startup combustor; supplying air to the startup combustor;igniting fuel supplied to the startup combustor; controlling a flowamount of air supplied to the startup combustor from the air supplydevice and a flow amount of fuel supplied to the startup combustor fromthe first fuel supply device, in response to the elapsed time afterinitiation of combustion in the startup combustor; stopping supplyingfuel to the startup combustor after a first predetermined time haselapsed after initiating combustion; and supplying fuel to the reformer.21. A fuel reforming system having a reformer for reforming fuel toproduce a reformate gas containing hydrogen (H₂) gas and carbon monoxide(CO) gas; a shift converter for reacting carbon monoxide contained inthe reformate gas with water (H₂O) to produce hydrogen; a CO oxidizerwhich removes CO gas from the reformate gas discharged from the shiftconverter and supplies the reformate gas to equipment using hydrogengas; a startup combustor for supplying combustion gas to the reformer,the shift converter and the CO oxidizer so as to warm-up the fuelreforming system; and a gas passage allowing flow of combustiongas/reformate gas, the gas passage extending to the equipment usinghydrogen gas from the startup combustor through the reformer, the shiftconverter and the CO oxidizer; the fuel reforming system comprising: anNOx trap disposed between the startup combustor and the equipment usinghydrogen gas, the NOx trap adsorbing nitrogen oxides in the combustiongas; first means for supplying fuel to the startup combustor; secondmeans for supplying fuel to the reformer; third means for supplying airto the startup combustor; fourth means for igniting fuel supplied to thestartup combustor; and fifth means for commanding the third means tosupply air to the startup combustor; sixth means for commanding thefirst means to supply fuel to the startup combustor; seventh means forcommanding the fourth means to ignite fuel in the startup combustor soas to initiate combustion in the startup combustor; eighth means forcontrolling a flow amount of air supplied to the startup combustor fromthe air supply device and a flow amount of fuel supplied to the startupcombustor from the first fuel supply device, in response to the elapsedtime after initiation of combustion in the startup combustor; ninthmeans for commanding the first means to stop supplying fuel to thestartup combustor after a first predetermined time has elapsed afterinitiating combustion, and tenth means for commanding the second meansto supply fuel to the reformer.